When you see those cool images of astronauts floating on the International Space Station,
you might think they’re enjoying the benefits of gravity disappearing in outer space. But there
is gravity in space. It’s the force holding the moon in its orbit around Earth, for one thing.
But even if you knew that gravity is exerting some force on astronauts, you might not have
realized that scientists orbiting earth are actually free-falling through space.
Hi, I’m Justin Dodd. Today, we’re breaking down misconceptions about physics, from how electrons
move to whether or not Isaac Newton ever actually got bonked in the head by an apple like some kind of big old
doofus-genius. Let’s get started.
If there’s gravity in space, how come astronauts fly around inside the International Space Station
like it’s a game of Pong? And is it still true that you can’t eat sandwiches in space because
the crumbs would float into the machinery? The amount of gravity keeping the universe
together is actually pretty small. It’s enough to harness massive celestial bodies in their orbits,
but it’s much less than what we recognize on our own planet. In the ISS, which is not so far from
our planet, relatively speaking, the force of gravity is about 90 percent of what it is on
Earth’s surface. So if you’re looking to lose 10 or 20 pounds, have I got a destination for you!
It's right here. In my arms. You're perfect just the way you are.
Astronauts there appear to float not because there’s less gravity; it’s because they’re
actually free-falling, meaning that their motion is caused only by gravity and no other force.
As Galileo figured out (more on him later), in the vacuum of space, all objects fall at the same rate
of acceleration, regardless of their mass. But the ISS doesn’t come crashing to the ground because,
while it’s falling towards Earth, it’s moving horizontally nearly five miles every second.
The Earth is curving away from the orbiting space station, meaning it can’t hit the ground.
And since both the ISS and the people in it are falling at the same speed, they float!
Even crumbs from a hypothetical sandwich would be falling at the same rate as a computer weighing
hundreds of pounds, and each would appear weightless. Special shoutout to YouTube commenter
Mark McCombs for mentioning that fact about the ISS on our misconceptions video about dogs. We
have an upcoming episode discussing misconceptions about prison. If you know any interesting myths
about that topic, you know what to do.
You know the famous tale: the physicist
Isaac Newton was sitting under a tree minding his own business when an apple fell on his head.
Newton immediately realized there must be some cosmic force that makes an apple fall
straight down to the earth, and possibly a more metaphysical entity that had damned him to a life
of fruit misfortune. Right then and there, he devised the theory of gravity.
However, like George Washington chopping down a cherry tree and being unable to lie about it,
Newton’s story is something of a myth. So what really happened?
After his university closed in 1665 due to a plague outbreak, Newton returned to his rural
hometown of Woolsthorpe, England. Moving home after a deadly pandemic? Who could relate. After
a long day of doing math problems, he liked to stretch out under the family’s apple tree.
According to his biographer William Stukeley, that is where the eureka moment occurred—but
it didn’t involve getting bonked on the head. Newton himself told Stukeley what had gone down
when they had dinner together on April 15, 1726. Newton was then the world’s leading physicist and
the president of the Royal Society. After the meal, Stukeley wrote in his notes, “the weather
being warm, we went into the garden and drank [tea], under the shade of some apple trees … he
told me he was just in the same situation, as when formerly, the notion of gravitation came
into his mind. It was occasion’d by the fall of an apple, as he sat in a contemplative mood.”
Newton had wondered to himself why the apple would always fall perpendicularly to the ground, and not
drift sideways or upward, or float on the breeze. He thought there must be an invisible force or
forces that not only put the apple in motion, but also governed its trajectory and speed.
The apple tree episode was just the start of his experiments and calculations toward solving this
puzzle. In the following two decades, he worked on the problem while also serving as a professor of
mathematics at Trinity College, Cambridge. In that time, he improved the reflecting telescope, which
concentrates light to allow greater visibility of far-away objects, and then presented his research
in optics to the Royal Society. He also sent a preliminary paper on motion to the astronomer
Edmond Halley (of comet fame), who had proposed that comets do not travel in straight lines but in
elliptical orbits around a focal point. I would’ve told Newton to stop making everyone else look
lazy, but Halley encouraged Newton to consolidate his theories of force and motion into a book.
The result, titled The Mathematical Principles of Natural Philosophy, broke science when it
was published in 1687. Newton laid out his three laws of motion, which describe inertia, momentum,
and actions and reactions, as well as his theory of universal gravitation. Newton posited that
inertia would make the moon fly out of its orbit around Earth in a straight line unless
Earth exerted a force on it, keeping it in its elliptical path. Long story short,
Newton concluded that Earth’s force on the moon is the same force that pulls apples off a tree—which
we now know as gravity. But the whole bonking thing… that's probably just a modern addition to
make kids giggle in science class.
You probably remember a diagram from your
high school physics or chemistry class. It shows electrons traveling around the nucleus of an atom,
kind of like a planet travels around the sun. The little ellipses make it
look like a train track that the electron rides. But that’s not really an accurate representation
of how electrons move. There might not even be a way to accurately represent how electrons move.
In 1913, the Danish physicist Niels Bohr published his model of an atom’s structure.
It had positively charged protons and non-charged neutrons held together in the atom’s nucleus, plus
negatively charged electrons circling the nucleus in concentric orbits, called shells.
When an electron hopped from an interior, lower energy shell to an outer, higher energy shell,
it absorbed a finite amount of energy; when the move was reversed, the electron gave
off a specific amount of energy. This is the model you probably remember from high school.
It’s also called the planetary model, because it mimics the movements of planets around the sun.
The problem is, the developers of quantum physics eventually realized that electrons don’t exist
in this straightforward way. Remember: when you get to a small enough scale,
things start to behave a lot differently than the way we’re used to thinking about objects in
the human-sized physical world. Electrons are quantum objects,
meaning they have a dual nature and can act like either a particle or a wave. They’re not solid,
discrete objects. Instead of moving in neat circles, electrons linger around the nucleus as
waves, called orbitals, that vibrate at different frequencies. They don’t have defined trajectories
or positions at all. Heisenberg’s Uncertainty Principle tells us that we can’t simultaneously
define their position and speed—just forget about trying to make a date for coffee with an electron.
....speaking from personal experience.
Rather than putting an electron at a specific place in a diagram, like a city on a map, it’s
more accurate to think of a range of probabilities where an electron could theoretically be.
To make matters even more complicated, believe it or not, there’s an exception to this indeterminate behavior. These
probabilistic, cloudlike orbitals can glom together into things called “wave packets.”
And according to a study from 2004, the packets can orbit the nucleus in a way that is actually
not dissimilar to the high school textbook diagram. Now, at this point, we’re getting into
physics that are honestly way above my pay grade. Let’s just leave it at this: If your conception
of individual electrons has them zooming around in a nice little ellipse, you should know that’s
not quite right. The actual representation… my brain hurts, just trust me on this one okay?
It’s like a scene out of a movie—a movie about experimental physics, which are disappointingly
lacking in modern Hollywood. Galileo climbs to the top of the Leaning Tower of Pisa and
drops two spheres of different weights to test the belief the heavier one will hit the ground first.
Shockingly, the spheres land at the same time. The reason? The heavier ball has a greater mass,
and because of that, gravity acts on it more forcefully. But thanks to inertia the heavier
ball needs more force to get it to accelerate. And weirdly those two effects cancel each other out.
Therefore, two objects of different masses will fall at the same speed in a vacuum.
But thankfully for us and annoyingly for physics we don’t live in a vacuum, so we
need to look at a third force—air resistance. Let’s say Galileo drops a big piece of plywood
and a brick out the window. Rude. But, ya know, I guess in the name of science. The plywood has
more mass and therefore more gravitational force than the brick, but as both fall,
the wood’s shape experiences more air resistance, so the brick should hit the ground first.
But getting back to the spheres … Galileo heaving them off the tower
probably didn’t happen. The legend can be traced to a biography of the scientist written by his
secretary and disciple Vincenzo Viviani, who, uh, embellished a few things. In addition to
the tower story, Viviani attributes Galileo’s discovery of the principles of pendulum motion
to his observation of a swinging lamp in Pisa Cathedral, which also likely didn’t happen. Those
poetic origin stories proved popular; the fanciful details were repeated by Galileo’s followers.
These falling-body experiments didn’t end with Galileo, though. They became popular public
scientific displays where non-scientists could see marvelous demonstrations of physical laws,
none more memorable than astronaut David Scott reenacting Galileo’s theory on the moon. During
the Apollo 15 mission in 1971, Scott set himself up in front of a TV camera on the lunar terrain.
He held a hammer in his right hand and a falcon feather in his left. As the camera zoomed out,
Scott dropped both objects at the same time, and both fell to the moon’s surface at the same time
in the near-vacuum of space—despite their different masses and shapes.
The moon is no leaning tower of Pisa but ,you know it’ll do.
In classical physics, predating Schrodinger and his contemporaries,
'particles' and 'waves' were thought of as two distinct categories of atomic existence. Particles
could be identified in space and time; they had qualities like mass and volume. No two particles
could be in the same place at the same time. Waves, on the other hand, could be spread out
in space, like sound waves being heard by different people in different places.
The physics that replaced this classical model recognized that reality is much more complex.
In a nutshell, quantum mechanics analyzes how matter behaves on the atomic and even
subatomic levels. It describes the actions and qualities of atoms and subatomic particles,
such as protons, neutrons, and electrons, as they interact with things like
electromagnetic radiation. This model holds that any particle can behave as a wave,
and vice versa. Schrödinger’s equation recognized this duality of atomic bits of matter—and
in an important breakthrough, it demonstrated the quantum behavior of wave functions. That is,
in order to describe waves, the most accurate mathematical model can only show the probability
of certain events happening, not the kind of fixed information we're used to analyzing in
the larger physical world. If a quantum system is not observed or measured, Schrodinger claimed,
then it does not have definable properties. You may not be surprised to hear that discoveries
about atomic behavior following Schrödinger’s equation struck scientists as kind of illogical. The
theories might be correct on the atomic level, but in the normal world, they make no sense.
Even Schrödinger himself was annoyed by this paradox. In 1935, he proposed a hypothetical
situation to illustrate its absurdity. A cat is locked in a steel box with a “tiny bit of
radioactive substance.” Over the course of an hour, if the substance decays, it will trigger
the release of a poison that will kill the cat. If the substance does not decay, the cat will live.
Both scenarios are equally probable. But, Schrödinger wrote, one will not know if the
cat is alive or dead without opening the box and looking inside, thereby observing the whole system
and defining its properties as either living or dead. Until the box is opened, the cat is both.
The hypothetical experiment showed how the laws of quantum physics don’t always line up with reality,
and physicists still debate the nature of the quantum world compared to the world
experienced at the scale we’re familiar with. Schrödinger was not advocating that anyone
actually try this experiment, and fortunately, no one has. But why a cat? His reasons for choosing
a cat for this exercise, and not a dog or hamster, are not clear. But he did grow up with an aunt who
owned as many as 20 Angora cats, so there’s that.
1905 was a banner year for Albert Einstein. The German theoretical physicist had earned
his doctorate and published four major papers in a German research journal. Though each
one described a revolutionary new approach to understanding aspects of the universe,
the final paper demonstrated the equivalence of mass and energy and summed up the complex
theory in the iconic equation, E=mc2. Unfortunately, Einstein’s discovery had an
unintended effect. It spurred European research into using the equivalence of
energy and mass to develop atomic weapons. In 1939, German chemists Otto Hahn and Fritz
Strassman (with Austrian physicist Lise Meitner) announced they had achieved nuclear fission.
Soon after, Einstein was contacted by the Hungarian physicists Leo Szilard, Eugene Wigner,
and Edward Teller. Szilard expressed their concern that German forces could potentially seize
Belgium’s supply of uranium, which it mined in its African colonies, for atomic weapons. Knowing
that Einstein was friendly with the Belgian Queen, Sziliard wanted him to sign off on a letter asking
Belgium to stop selling uranium to Germany and give it to the United States. Einstein suggested
sending it to a cabinet minister instead, and that was that. It was quickly realized that a
letter should be sent to Roosevelt about the threat, which Einstein also signed.
An envoy hand-delivered the letter to President Franklin Roosevelt and even read it out loud to
make sure the commander-in-chief understood the precipice on which humanity stood.
The signature of Einstein, the world’s most famous scientist, undoubtedly helped
convey the importance of the matter. Roosevelt approved the research and
the funding to launch what was essentially the predecessor of the Manhattan Project. Years later,
Los Alamos National Laboratory director J. Robert Oppenheimer led a team of scientists, including
physicist Enrico Fermi and nuclear chemist Glenn Seaborg, with oversight from the U.S. Army.
Einstein never worked on the Manhattan Project and was denied a security clearance because of his
pacifist stance. The project eventually developed and tested the world’s first nuclear weapons.
The only two atomic bombs ever deployed in warfare landed on Hiroshima and Nagasaki, Japan, in 1945.
By then, Szilard was writing petitions AGAINST using America’s nuclear weapons. Ironically,
the Axis Powers never got close to mastering the technology for their own nuclear weapons.
Einstein regretted his role, however small or large, in promoting nuclear development. As he
said, “Had I known that the Germans would not succeed in developing an atomic bomb,
I would have done nothing for the bomb.”
Thanks for watching Misconceptions. Physics is a bit confusing for my worm brain, but at least it’s something I can blame whenever I
drop my phone and crack the screen for the 11th time. I’ll see you next time. Damnit physics.